A2 Chem Unit 4: Isomerism

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Transcript of A2 Chem Unit 4: Isomerism

Chiral compounds Geometric isomers Non-superimposablemirror images Optical isomers are NON-SUPERIMPOSABLE MIRROR IMAGES. Non-superimposable mirror images Look at each carbon atom in the chain and see what is attached to it. For a chiral centre you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL The answers Geometric isomers Types of isomers Homologous series (AS) UNIT 4 4.5 Organic chemistry II

Lesson 1 – Isomerism A2 Chemistry EDEXCEL 9080 If a molecule has 2 chiral centres then there are 4 possible optical isomers. The extent of rotation of plane polarised light is not the same.

If a molecule has two identical chiral centres, the isomer with a plane of symmetry will not be optically active because each chiral centre will rotate light in opposite directions. Two chiral centres The angle through which the plane of polarisation is rotated depends on:

The nature of the enantiomerThe concentration of the enantiomer in the solution. This is a useful way of measuring the concentration during a reaction – the extent of polarisation is proportional to concentration. Further properties of optical isomers A solution containing equimolar amounts of the two enantiomers is called a racemic mixture. It does not rotate the plane of polarisation of plane-polarised light. Racemic mixtures Light waves have peaks and troughs in all directions. When it is passed through a polariser (a piece of Polaroid), the light that comes out only has peaks and troughs in one direction. The light have been polarised.If this polarised light is passed through one enantiomer then the light is rotated in one direction (the other enantiomer will rotate it in the opposite direction). Polarised light Enantiomers have identical chemical properties and the same boiling temperatures and solubilties.They differ in two ways:They rotate the plane of polarisation of plane-polarised monochromatic light.Optical isomers have different biochemical reactions., e.g., glucose, CHO(CHOH)4CH2OH is one of 16 optical isomers. Glucose is the only one of the 16 that can be metabolized by humans. Properties of optical isomers 1-chloro-2-methylpropanane (CH3)2CHCH2Cl 2-chloro-2-methylpropanane 2-chlorobutane 1-chlorobutane (CH3)3CCl CH3CH2CH2CH2Cl CH3CH2CHClCH3 Identify the chiral centre (or chiral carbon), if any. Spot the chiral centre another form of stereoisomerism occurs when compounds have non-superimposable mirror imagesthe two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it. Optical isomerism The C-C bond has free rotation and so the above isomers are all the same. Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same. A single C-C can rotate Exam questions on organic chemistry often focus on questions regarding the identification of functional group isomers given details about their chemical reactions. Molecular formula, C3H6O propanone propanal Aldehydes and ketones Structural – functional group halogenoalkanes Structural isomers – positional A systematic name has three parts: IUPAC naming rules In this lesson you will:

Understand the concept of homologous series and apply the IUPAC nomenclature for compounds.Learn about the different functional groups and how to recognise and name them in compoundsRecognise chain, positional and functional group isomerismRecognise stereoisomerism: geometric (cis-trans) and opticalUnderstand optical isomerism arises from a chiral centre and that optical activity can be determined by the polarisation of plane polarised monochromatic light and understand the nature of a racemic mixture. Introduction to organic chemistry Topic 4.5 – Lesson 1 A polarimeter is used to measure the rotation of the plane of polarisation of plane-polarised light. Polarimeter same Geometric isomers are only found in the alkenes and their derivatives. The isomers arise because, unlike a single C-C bond, there is no rotation around the C=C double bond. The isomers have different physical properties (e.g., bp) but, generally, similar chemical properties. Stereoisomerism - geometric 2-hydroxypropanoic acid What is the systematic name of lactic acid? Lactic acid, CH3CH(OH)COOH Examples This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space. C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two. Double bonds C=C cannot rotate Example used is Butane The full structural formula or displayed formula shows all the bonds present in the molecule Structural formula: the arrangement of groups of atoms in the molecule. Molecular formula: the exact number of atoms of each element in a molecule Empirical formula: the simplest ratio of atoms in a molecule CH3CH(CH3)CH3 CH3CH2CH2CH3 C2H5 C4H10 Types of formulae Fewer van der Waals forces – reduced surface area Why does the branched structure have a lower boiling point? Chemical properties are the same since the functional group is the same. However, the branched structure has a lower boiling point. e.g., Butane, C4H10, has two chain isomers Structural isomers - chain Isomerism